The present invention relates generally to methods and apparatus for encoding data into the vertical intervals of a plurality of television signals. The invention more particularly relates to methods and apparatus for encoding data into the vertical intervals of a plurality of simultaneously transmitted subscription or cable television signals, the vertical intervals of the various television signals typically occurring in an unsynchronized manner.
The vertical intervals of television signals are finding increased use as a medium for transmitting various types of data. For example, teletext and captioning-for-the-deaf data is normally transmitted during selected horizontal lines of the vertical interval of a broadcast television signal. Also, the vertical interval of a broadcast television signal which has been scrambled to limit access thereto is frequently used to transmit appropriate instructions for enabling an authorized receiver to unscramble the signal.
In one known system, selected data is encoded in the vertical intervals of a plurality of television signals by a facility located intermediate the actual sources of the signals and the signal reproducing television receivers. Thus, in a cable television (CATV) or subscription television (STV) system, the head-end facility typically receives a plurality of television signals from various sources, e.g. satellite broadcasts, encodes selected data into the vertical intervals of the received television signals and then simultaneously re-broadcasts the encoded television signals over multiple television channels. In a system of this general type, the vertical intervals of the television signals received by the head-end facility are normally not synchronized in time so that precautions must be taken to avoid conflicts between the encoding and re-broadcasting operations. That is, in order to prevent data loss or degradation, the head-end facility must be operated such that, for each channel, a vertical interval used to re-broadcast encoded data is not at the same time being updated with new data.
Prior art techniques for accomplishing the foregoing typically utilize some sort of double buffering arrangement. In such arrangements, each channel includes an output register supplied by an intermediate buffer register which is in turn supplied by a common computer. Selected data is transferred from the computer to each buffer register at the television signal field rate and stored therein until the contents of the associated output register have been read during a vertical interval of the television signal transmitted by the respective channel. At this time, the data stored in the buffer register is transferred to the output register while the buffer register is updated with new data from the computer. The contents of the output register are again encoded into the subsequently occuring vertical interval of the television signal after which the output register is again updated from the buffer register and the buffer register from the computer. As explained previously, this double buffering arrangement is necessary in order to prevent data loss/degradation which could otherwise result by encoding a vertical interval of the television signal with data from the output register while, at the same time, attempting to update the output register with new data. The fact that the vertical intervals of the television signals received by the head-end facility occur in an unsynchronized and unpredictable manner make the need for data transfer characteristics such as exhibited by the double buffering arrangement even more important.
While the double buffering arrangement described above generally provides satisfactory results, the expense and complexity of the overall system are considered to be distinct disadvantages thereof. In this regard, although only a single computer is required to initially generate the selected data, a buffer register together with appropriate logic to control the transfer of data from the buffer register to the output register is required for each channel of the system.
It is accordingly a basic object of the present invention to provide improved methods and apparatus for encoding data into the vertical intervals of a plurality of received television signals.
It is a more specific object of the invention to provide improved methods and apparatus for encoding data into the vertical intervals of a plurality of received television signals without any concomitant data loss or degradation.
It is a further object of the invention to provide an implementation of a system of the foregoing type which is relatively inexpensive and non-complex and which does not require the use of intermediate buffer registers.